This invention relates to a surface mount type capacitor to be mounted on a circuit board and mainly used as a noise filter.
Recently, along with performance improvements, a downsizing, and reductions in weight in electronic devices, the downsizing of a power supply using such electronic devices therein has been rapidly in progress. The power supply can be further downsized by heightening the working or operating frequency thereof.
With a rise in the operating frequency, a countermeasure against noise induced has to be taken. Among various components used in power supply, the performance requirements for a noise filter have been increasingly becoming severe. Such requirements also come from the utility devices because, with a rise in the switching rates of digital electronic devices, it is increasingly needed to decouple a wide range of high frequency components generated in the devices from power lines.
As such a noise filter, a surface mount type capacitor employing solid electrolytic material has been developed and put into practical use. Such a surface mount type solid electrolytic capacitor has an anode made of valve function metal such as aluminum and a cathode made of a functional polymeric material having high electrical conductivity as a solid electrolytic material. The solid electrolytic capacitor has an ESR (Equivalent Series Resistance) of 1/20 to 1/50 of that of the nonsolid electrolytic capacitor such as an aluminum electrolytic capacitor or tantalum electrolytic capacitor.
As such a surface mount type solid electrolytic capacitor, there is a capacitor having a structure of a distributed constant transmission line type and three terminals. This capacitor is often called a three-terminal or a triode noise filter. The capacitor has an anode section, a cathode section, first and second anode terminals, and a cathode terminal. The anode section is provided with an anode portion and first and second leads. The cathode section comprises a cathode formed on a solid dielectric film over a peripheral surface of the anode section except the first and the second leads, respectively. The first and the second anode terminals are electrically connected to lower sides of the first and the second leads, respectively. The cathode terminal is electrically connected to a lower side surface of the cathode section.
Use is made of the capacitor as a noise filter in such a way that the first and the second anode terminals are electrically connected between a DC power line of a utility device and a power supply line of a power supply on a circuit board. The cathode terminal is electrically connected to the ground of the circuit board.
High frequency components of voltages or currents appearing in either the DC power line of the utility devices or the power supply line of the power supply are filtered and eliminated through the capacitor, resulting in substantially noise-reduced voltages or currents in the opposite anode terminals.
However, in case that such a surface mount type capacitor is used in a high frequency region such as more than 100 MHz, an electromagnetic wave noise may be radiated from the first or the second anode terminal. The radiated noise may be transmitted in the resin package and the air. The propagated noise may attain to and may be input into the second or the first anode terminals. This is because the first and the second leads are electromagnetically exposed. The input noise interfere with and deteriorates the noise-filtering performance of the capacitor.
As a solution of the above-mentioned disadvantage, Japanese Unexamined Patent Application Publication (JP-A) No. 2002-313676 by applicant discloses a surface-mounting type capacitor having a metal plate which is electrically connected to an upper side of a cathode section and covers first and second leads. The metal plate serves to suppress or control the behavior of the propagation of the electromagnetic wave noise.
The disclosed surface mount type capacitor can prevent the electromagnetic wave noise considerably but a further noise reduction is desirable in a higher and wider frequency range with the increased operating frequency of the utility devices.